1. Field of the Invention
The present invention relates to an optical pickup device that writes and reads signals to and from an optical recording medium by irradiating light to the recording medium, and an optical disk drive that drives the optical pickup device.
This application claims the priority of the Japanese Patent Application No. 2003-092346 filed on Mar. 28, 2003, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
The optical pickup device irradiates light to an optical recording medium, and detects return light from the optical recording medium. FIG. 1 schematically illustrates the optical system of a conventional optical pickup device. The optical pickup device is generally indicated with a reference 90. As shown, it includes a semiconductor laser 91 as a light source, an objective lens 93 that focuses light from the semiconductor laser 91 on an optical recording medium 10, a photodetector 97 that detects return light from the optical recording medium 10, etc.
The above optical pickup device 90 is built in an optical disk drive, and writes signals to the optical recording medium 10 and reads signals recorded in the optical recording medium 10.
In the optical disk drive, the optical output from the semiconductor laser used as a light emitting element, for example, has to be varied depending upon whether information signals are to be read from an optical recording medium or written to the latter. Also, the optical output has to be varied depending upon the type of an optical recording medium used, multilayer type or high-speed read/write type, and the read/write speed of the optical recording medium used, high, medium or low.
Normally, for determination of an efficiency with which light emitted from a semiconductor laser is to arrive at an optical recording medium (the efficiency will also be referred to as “efficiency of light utilization” hereunder wherever appropriate), the semiconductor laser is first set for an optical output of 3 mW or more. If the optical output of the semiconductor laser is less than 3 mW, the laser noise of the semiconductor laser will increase drastically to degrade the quality of signals read from the optical recording medium. On the other hand, the intensity of light arriving at an optical recording medium will depend upon the characteristic of the optical recording medium. It is 0.3 mW, for example. In an optical pickup device using the above-mentioned semiconductor laser and optical recording medium, when the semiconductor laser is set for an efficiency of light utilization of 10%, the semiconductor laser provides an optical output of 3 mW and the light arriving at the optical recording medium has an intensity of 0.3 mW for reading from the optical recording medium. For writing to the optical recording medium, the intensity of light arriving at the optical recording medium also depends upon the characteristic of the optical recording medium. For example, for an optical recording medium needing a light intensity of 0.3 mW for reading and a light intensity 10 times higher than that for reading, the light intensity required for writing to the optical recording medium is 3 mW. At this time, the semiconductor laser has to provide an optical output of 30 mW for writing to the optical recording medium. On the assumption that the optical recording medium is of a single-layer, normal-speed type, the semiconductor laser has to provide an optical output of 60 mW for a light intensity of 6 mW, for example, required for writing to a two-layer optical recording medium. Further, on the assumption that the optical recording medium is of a two-layer, double-speed type, the semiconductor laser has to provide an optical output of 120 mW for a light intensity of 12 mW required for writing to a two-laser optical recording medium.
For writing signals to such various types of optical recording media, the semiconductor laser has to provide a higher output. However, such a higher semiconductor laser output will bring about a lower reliability, edge breakdown, shorter life, etc. of the semiconductor laser. Further, the semiconductor laser will consume more power and run a higher temperature.
To solve the above problems, it has been proposed to use an attenuator. For reading signals from the above-mentioned optical recording medium with the semiconductor laser being set for an efficiency of light utilization of 20%, for example, the attenuator is activated to halve the necessary light intensity. Thus, the attenuator enables recording with a half of the semiconductor laser output.
There have been proposed attenuators using a liquid crystal or diffraction element. In the attenuator using a diffraction element, for example, zero-order light is attenuated by making a light intensity modulation of diffraction efficiency by the diffraction element and shielding the diffracted (cf. the Japanese Patent Application Laid-Open No. H09-223328).
Also, it has been proposed to attenuate the zero-order light by a light intensity modulation of the diffraction efficiency by the diffraction element and varying the ratio in light amount between ±first-order light used for detection, by three beams, of servo signals and the zero-order light used for detection of return light from the optical recording medium (cf. the Japanese Patent Application Laid-Open No. 2002-90784, for example). This method is different from the light attenuation for reading and writing.
In the above method disclosed in the Japanese Patent Application Laid-Open No. H09-223328, however, to shield the diffracted light, the diffraction angle has to be increased or the aperture provided on the objective lens and the diffraction element have to be spaced sufficiently from each other. For a sufficiently large diffraction angle, the diffraction-grating interval has to be increased sufficiently, which will lead to a difficulty in making the diffraction element. Also, for an increased distance between the aperture on the objective lens and the diffraction element, the optical pickup device has to be designed larger.
On the other hand, the method disclosed in Japanese Patent Application Laid-Open No. 2002-40784 has such a problem that increasing the intensity of the ±first-order light to over that of the zero-order light results in erasure of signals recorded in the optical recording medium because since the modulated diffracted light is used to detect light. Also, since the ±first-order light returning to the photodetector varies in intensity as well, the photodetector has to have the detectivity range thereof increased.
Also, an optical attenuator using a liquid crystal has been proposed. In this attenuator, the polarized direction of light passing through the liquid crystal is modulated to control the amount of light passing through a polarizing beam splitter provided downstream of the liquid crystal. However, this element has to be provided in parallel light. Insertion of the element in the parallel light will cause the element itself to be larger than that element inserted in divergent light. Also, in a small optical pickup device using an optical lens commonly for both collimation of light emitted from a semiconductor laser and condensation of the light to a photodetector, the coming light and going light will pass through the attenuator, which will result in attenuation of the light focused to the photodetector.